The present invention concerns a capacitive pressure detector. The invention also concerns a method for manifacturing such a detector.
In respect of the prior-art technology, reference is made to the following papers:
[1] C. S. Sander, J. W. Knutti, J. D. Meindl, IEEE Transactions on Electron Devices Vol ED-27 (1980) No. 5, pp. 927-930 PA0 [2]U.S. Pat. No. 4,261,086 PA0 [3]U.S. Pat. No. 4,386,453 PA0 [4]U.S. Pat. No. 4,384,899 PA0 [5]U.S. Pat. No. 4,405,970 PA0 [6]U.S. Pat. No. 3,397,278.
In cited publication [1], a capacitive detector of absolute pressure is described, which consists of an elastic element of silicon and of a glass plate, which have been attached to each other by means of the method of cited paper [6]. Between the elastic element and the glass plate, a cavity remains which functions as the vacuum capsule of the detector. Between the elastic element and a metal film placed on the glass plate, a capacitance dependent on the pressure is produced. An electrical connection to the metal film placed on the glass plate is obtained from outside the detector by means of a conductor prepared into the silicon by means of diffusion and having a type of conductivity different from that of the elastic element. A major drawback of this detector is the high and highly temperature-dependent depletion capacitance produced between the diffused conductor and the elastic element, which enters to the side of the pressure-dependent capacitance of the detector. The relative dynamism of the detector decreases, and its dependence on temperature increases.
In the cited publications [2], [3]and [4], a pressure detector construction similar to the above is described. The wiring thereof is, however, different. The wiring is made through a hole drilled into the glass, which is metal-coated inside. The hole is closed by melting metal (solder) into it. The wiring has no parasitic properties. The sealing of the holes are, however, rather inconvenient to carry out in mass production.
In cited publication [5], a detector is described in which support plates made of silicon and the elastic element are "glued" onto each other by means of a thin glass film, which is made by sputtering or by vacuum evaporation. By means of the thickness of the glass film, the distance between the capacitor plates is also controlled. It is a good aspect of the detector construction that the material of manufacture is almost completely of silicon. That guarantees a good temperature stability. The stray capacitances related to the glass joint, however, spoil the properties of the detector. By means of the above methods, the thickness of the glass can be, at the maximum, 10 .mu.m, whose capacitance corresponds to an air gap of 2 .mu.m. Thus, the role of the joint zone is dominating in the capacitance of the detector, unless the area of the detector is very large.
In the cited publication [5], a construction is also described in which a high glass wall separates the said two silicon pieces. In that case, there is no problem of stray capacitance. The dimensional accuracy of the air gap in the capacitor will, however, be poor.
The object of the present invention is to eliminate the drawbacks present in the prior-art technology described above and to provide a capacitive pressure detector of an entirely novel type as well as a method for manufacturing the same.
The invention is based on the idea that a layer of an insulating material, e.g. glass, is melted or cast onto a substrate of a machinable and conductive material, e.g., silicon. Thereby, the thickness of the insulation layer is higher on the recesses provided in the substrate than on other portions thereof. When the insulation layer (and the substrate, partly) are ground so that the topmost portions of the frame layer and the remaining portions of the insulation layer form a uniform plane face, an electric passing-through is provided that extends from the bottom face of the substrate layer to its top face and that is, at its top end, surrounded by an insulation layer. Through the electric passing-through, it is possible to make a mechanical passing through by machining a hole through the electric passing-through.